1. Technical field
This invention is in the field of magnetic sensors which detect a magnetic field or magnetic field change and find use in linear or rotary motion detection, electrical current sensing, linear or rotary position sensing, magnetic imaging, magnetic recording read heads, magnetic recording media as well as general magnetic field sensing.
2. Background of the Invention
Because of the many applications of magnetic field sensors, there is a long history of technical development of materials and means to measure fields of various strengths. No one sensor can perform every function well. Factors such as size, weight, power consumption, and cost should be minimized by a field sensor. Sensitivity, linearity, bias, stability, reliability, and operating temperature and frequency range are factors that should be optimized. As with any instrument, it is usually difficult to achieve all of these characteristics in one device. The most common magnetic sensors used in a variety of applications are the Hall effect sensor and the variable reluctance coil. The drawback of variable reluctance devices is that they generate signals proportional in size to the time rate of change of magnetic flux. The signal size therefore decreases with decreasing speed, and below a certain flux change rate, the signal disappears into the noise. Hall effect devices generate a very small raw signal because of low field sensitivities (0.5xcx9c5 mV/100 Oe applied field). This mandates signal conditioning, and requires that a certain minimum field be available for device operation.
The concept in combining the magnetostrictive materials and piezoelectric layers for highly sensitive magnetometer was first introduced by Mars D. Mermelstein in 1986. In his patents U.S. Pat. No. 4,769,599 and U.S. Pat. No. 5,130,654 a magnetometer is disclosed as a device using a piezoelectric resonator to create a standing wave in a sensing magnetostrictive ribbon and using a pickup coil to read out the electromotive force. A minimum detectable field gradient of 7.7 pT/cm-Hz was achieved in this device by using a differential amplifier technique. In October 1997, a device called piezomagnetometer was patented by Walter N. Pondney. In that device a stack of 201 alternating piezoelectric and magnetostrictive layers is used, in which 100 pairs of piezoelectric-magnetostrictive layer capacitors are connected electrically in parallel to increase the charge storage by raising the effective capacitor plate area. The device requires multiple layers that are placed in a permanent biasing magnetic field normal to the layer surface. A resolution as high as 1 pT/cm-Hz is achieved.
Our invention represents a new technology of passive solid-state magnetic sensors based on a combination of magnetostrictive material and piezoelectric material.
The innovative sensors can be mass produced at low cost in comparison with any existing magnetic sensor technologies including variable reluctance coils, Hall-effect devices, magnetoresistance semiconductors, and the most recently developed giant magnetoresistance (GMR) metal multilayers. Such a magnetostrictive/piezoelectric sensor does not consume any electrical power and easily has a field sensitivity larger than 10 mV/Oe. The devices including digital speed meter, digital flow meter, and digital electrical current meter have been demonstrated as application examples of the inventive sensors.
The combination of piezoelectric and magnetostrictive layers has not yet been used for the general applications of magnetic sensors. The present invention is basically a device that has two magnetostrictive layers and one piezoelectric layer or two piezoelectric layers and one magnetostrictive layer. Three-layer devices can be connected electrically in series to increase the voltage by raising capacitor plate separation. The sensors are designed for general magnetic field detection purposes in applications ranging from speed, flow, and electrical current detection to the information storage and imaging. The advantages of the inventive sensors over the competitive technologies are passive solid-state, high field sensitivity, wide dynamic range up to several thousands of Oersted, and low-cost in manufacturing.
Generally, the present invention uses a piezoelectric material in contact with a magnetostrictive material. The magnetostrictive component strains in response to presence of a magnetic field. This strain couples to the piezoelectric material, causing this material to produce an electrical output signal. The main objective of the invention is to provide an innovative, low cost and highly sensitive magnetic sensor of the simplest design, which requires no electrical power for the sensing element. The sensitivity of the device and the operating magnetic field range can be adjusted through material properties and structure designs for a variety of applications. The important variables used in the design include 1) selecting magnetostrictive materials with appropriate properties, 2) selecting piezoelectric materials with optimal properties, 3) determining the optimal number of capacitive elements, 4) selecting the appropriate thickness of each layer, 5) designing the geometry of the structure, and 6) establishing the most efficient bonding and packaging methods. The inventive magnetic sensors can be widely used in replacement of Hall-effect sensors, variable reluctance coils, and magnetoresistive devices. The particular applications of the inventive magnetic sensors are (a) speed detection and controls for rotary machines including automobiles, airplanes, locomotives, etc., (b) flow meters for reading and controls of liquid or gas flows, (c) electrical current meters for reading and controls of electrical power usage, and (d) micromagnetic field sensors for magnetic recording/reading heads and magnetic imaging devices.